1. Field of the Invention
The present invention relates to a package base on which a semiconductor element and one or more other elements such as optical elements can be mounted. The present invention also relates to a semiconductor package in which a semiconductor element and one or more other elements such as optical elements are mounted on a package base. For example, the semiconductor element is an LED (light emitting diode), an LD (laser diode), or a PD (photo diode), and the optical elements are an optical fiber, an optical crystal, a lens, and the like.
2. Description of the Related Art
Package bases (including heatsinks, stems, carriers, and the like) for semiconductor light-emitting elements, semiconductor light-receiving elements, and the like are required to satisfy the following conditions:                (i) The package bases have high heat conductivity and temperature controllability.        (ii) Portions of the package bases in contact with the semiconductor elements have thermal expansion coefficients close to the thermal expansion coefficients of the semiconductor materials so that the package bases do not impose great thermal stress on the semiconductor elements.        (iii) The material cost is sufficiently low.        (iv) Assembly is easy, and high positioning precision can be achieved.        (v) Electrodes wiring for driving the light-emitting elements and the light-receiving elements is possible.        (vi) Bonding does not cause deterioration of characteristics of the semiconductor light-emitting elements, semiconductor light-receiving elements, and the like.        
The Japanese Unexamined Patent Publications Nos. 8 (1996)-111481 and 10(1998)-107190 disclose use, as a heatsink, of a member being made of a carbon-fiber composite material and having a tabular form, where the member used as the heatsink is formed so as to have a flat mounting surface for a semiconductor element. The carbon-fiber composite material has high heat conductivity, and enables easy temperature control. In addition, since the carbon-fiber composite material has a thermal expansion coefficient close to the thermal expansion coefficients of the semiconductors, the thermal stress imposed on the semiconductor elements can be suppressed.
However, the methods disclosed in the above patent publications require at least two bonding processes. That is, a semiconductor element (e.g., a semiconductor light-emitting element, semiconductor light-receiving element, or the like) is bonded to a heatsink in the first process, and the heatsink is bonded to another member on which electrode connection portions (electrode terminals) and the like are formed, in the second process. Therefore, the mounting precision deteriorates and the manufacturing cost increases.
In addition, the Japanese Unexamined Patent Publication No. 10(1998)-107190 discloses a semiconductor package having a layered structure comprised of a semiconductor element, a buffering member (heatsink), and a metal radiation plate, which is characterized in that the buffering member is a plate made of a carbon-fiber composite material. However, when a plurality of elements, e.g., a semiconductor element and an optical waveguide, are mounted, alignment of the semiconductor element and the optical waveguide is difficult since the mounting surface of the buffering member (heatsink) is uniformly flat. Further, since terminals for electrodes and the like are not provided on the buffering member (heatsink), it is not easy to operate the semiconductor element on the buffering member (heatsink).
The Japanese Unexamined Patent Publication No. 8 (1996)-111481 discloses a heatsink for a semiconductor element, which is characterized in that the heatsink has a thermal expansion coefficient which is substantially identical to that of the semiconductor element mounted on the heatsink, and is made of a unidirectional carbon/carbon-fiber composite material which is heat conductive in the thickness direction. However, since the physical properties (including hardness) of the carbon fiber portions of the unidirectional carbon/carbon-fiber composite material are different from those in the other portions of the unidirectional carbon/carbon-fiber composite material, it is difficult to make the mounting surface a mirror-finished surface. In addition, although the heat conductivity in the thickness direction is high, the heat conductivity in the other directions is low. Therefore, the degree of freedom in designing the shapes of the packages and the like is small. Further, as in the case of JUPP 10(1998)-107190, when a plurality of elements, e.g., a semiconductor element and an optical waveguide, are mounted, alignment of the semiconductor element and the optical waveguide is difficult since the mounting surface of the buffering member (heatsink) is uniformly flat. Furthermore, since terminals for electrodes and the like are not provided on the buffering member (heatsink), it is not easy to operate the semiconductor element on the buffering member (heatsink).